1. Field of Invention
This invention relates to analog/digital converters and to shaft angle encoders, and more particularly to apparatus for providing a digital indication of a synchro rotor angle.
2. Description of the Prior Art
As is well known, there are many fields of applications for measuring the angle of a shaft or other rotary devices. To simplify illustration of the environment and background of the present invention, consider airborne systems, particularly those found on commercial and military aircraft. All existing airborne systems include rotary devices and means for determining the current angular position of such devices. For instance, synchronous generators, better known as, and referred to hereinafter as synchros, are utilized in many systems for a variety of purposes. In some cases, it is possible to incorporate a direct shaft angle encoder in a synchro or other rotary device. Such encoders are commonly configured so as to provide a direct digital encoding of shaft angle, such as by optical means. However, there are a great many applications where such direct encoding devices are not incorporated in existing equipment, and wherein a need arises to provide a real-time indication of the angular position of such devices. One example is the provision of flight data recorders of various kinds; included among these are what are sometimes referred to as "crash recorders" which provide information of key aircraft parameters in a crash-proof recording system in order to assist in the analysis of events leading up to a crash. Another example is in airborne integrated data systems, such as the type disclosed in a copending application of the same assignee entitled SELECTIVE DATA HANDLING APPARATUS, Ser. No. 803,372, filed on Feb. 28, 1969 by Provanzano, Jr. et al now U.S. Pat. Re: 28,109. In such a system, since a digital airborne computer is utilized to control multiplexing of the various parameters and to perform limit and other parameter testing of the parameters, a direct digital input is to be preferred. Further, the information recorded by such systems is usually analyzed on a ground-based computer in order to determine characteristics of the manner of flight and characteristics of the aircraft, for instructional and diognostic purposes. A common form of digital representation of angular position known to the prior art is the conversion to a digital value of the values of the sine and cosine of the position angle. However, this either requires special equipment to convert from sine and cosine to a digital representation of angle, or requires that both the value of the sine and cosine be stored digitally on board the aircraft, with a conversion to occur in a ground based computer system. In cases where direct, on-line usage of angular position is desired, it is of course necessary that angle be encoded on a real-time basis (or substantially so). Many position angle encoding devices known to the prior art require integration over many cycles, and thus defeat this purpose of real-time conversion. Also, high resolution and disambiguity of sine and cosine are difficult to achieve with prior systems.